In general, microdiagnostics are used to test a CPU. Usually, a console processor loads the test microcode and initializes the machine. The test microcode uses a bottom-up strategy to perform tests on the machine itself during its start-up.
The bottom-up strategy begins by performing the lowest level test such as a check on the console interface. If the test result such as a check on the console interface. If the test result is positive, the next higher level test is performed. As each test result returns positive, the microdiagnostic performs higher level tests, thus relying on previously checked parts of the machine to perform subsequent tests. For example, the microdiagnostics can initially check the microcode sequencer in the instruction box. If the sequencer is working properly, subsequent tests are performed on the execution box, floating-point box, caches, etc. utilizing the instruction box to perform the tests. By using the microcode to operate in the machine itself, a very fast test engine is developed as opposed to the use of a relatively slow console.
The use of microdiagnostics in conjunction with the cycle counter addressed the undesirably large CPU microdiagnostic test time problems that field service engineers, module repair centers, quality verification and initial stage manufacturing were experiencing. Any additional test time translates directly into extra costs for producing and maintaining a CPU.